I think it is based on the concerns some parents have regarding any autism risk involved. While many say that the vaccine has been proven safe it would be more acurate to state that there is no clear connection between autism and the vaccine and some studies show little correlation. There is no absolute proof that there is no connection.
There is also no proof that learning gemorah doesn’t cause heart attaks. What’s your point. The Dr. Who made the autism claim was a quack with serious conflicts of interest in the case. His claim is just as outrageous as the one I just made.